I think the sensor to detect are gone now I think the car figures it out (GM anyway) more or less by long term fuel trim. For the most part as long as your LTFT doesn't get to high you won't get a check engine light.
I blended some E85 Monday in my Cobalt XFE(non-ffv) and was around E55, after about 40 or so miles it threw a CEL, put in a couple more gallons of E10 when I could and brought it down to E45, light stayed on, next day 3 more gallons of E10 for E35 and the light cleared itself.
http://www.speedperf6rmanc3.com/cont...evel_Study.pdf
Interesting read on ethanol blends and non-FFV cars. I've had 50% blends in all my cars without any issue other than an occasion CEL